1. Field of the Invention
The present invention concerns a composition for use in removing urethane-based conformal coatings from printed circuit boards.
2. Background of the Invention
As the cost of sophisticated electronic chips continues to decline, many new applications have been found, including applications in adverse and challenging environments. Such chips are typically installed with other electronic components on printed circuit boards (PCBs). When PCBs are installed in adverse environments, their components must be protected. An important way of protecting PCBs is to dip or coat them in a material which forms, when cured, a resistant protective coating. Because such coatings conform to the general shape of the PCB components, they are often referred to as "conformal" coatings. One important, widely used type of conformal coating is polyurethane, such as that formed from moisture-cured urethane pre-polymer compositions.
An especially challenging new environment for PCBs is in the engine compartment of automobiles and other vehicles, where they guide operation of the engine and other automotive systems. Polyurethane conformal coatings have been used to protect automotive PCB from heat, humidity and other adverse environmental factors.
Servicing autos equipped with PCBs protected by conformal coatings presents a difficult problem. Where the operation of the vehicle has been impaired because of a damaged, defective or failed component on the PCB, even a very inexpensive component, the entire board is typically replaced, at substantial expense. The conformal coating makes isolation and replacement of the failed component difficult. Because the conformal coating is engineered to resist damaging environmental factors, it also typically resists chemicals otherwise effective at stripping coatings. Thus, very potent and potentially hazardous acids, bases, and/or organic solvents have been used to remove conformal coatings. Often removal is a very labor-intensive, and therefore expensive process, involving physically scraping the coating off the PCB. After a motorist's problem has been solved by replacing a failed PCB with a new one, the old PCB may be discarded although the defect may be very minor, and the value of the other components is lost.
Prior art stripping compositions for conformal coatings tend to employ highly toxic and/or highly corrosive materials. For example, U.S. Pat. No. 3,625,763 discloses a conformal coating stripping method and composition which includes a large proportion of a polychlorinated aliphatic liquid. U.S. Pat. No. 4,741,784 discloses a composition and method for removing conformal coatings such as polyurethane or epoxy from PCBs. A two-part composition is employed. The base component comprises toluene and/or methylene chloride. While conformal coating stripping compositions based on the less-toxic N-methylpyrrolidone are also available, they tend to act more slowly. U.S. Pat. No. 5,501,761 discloses a method of stripping conformal coatings from PCB using supercritical carbon dioxide. However, this method requires special high pressure processing equipment which is not generally available.
Thus, there is a need for a stripping composition which can rapidly remove urethane conformal coatings from PCBs, without requiring excessive labor, or the use of chemicals known to be hazardous, such as methylene chloride, or expensive equipment.